Nickel containing, rechargeable alkaline cells are presently in wide use in battery systems including nickel metal hydride batteries and the like. For purposes of this disclosure, the terms “batteries” and “cells” will be used interchangeably when referring to one electrochemical cell, although the term “battery” can also refer to a plurality of electrically interconnected cells.
While the present discussion focuses primarily on nickel metal hydride (NiMH) batteries, it should be understood that the electrode structures of the present invention can be used in other types of batteries which include nickel hydroxide based positive electrode materials. In general, NiMH cells employ a negative electrode made of a hydrogen storage alloy that is capable of the reversible electrochemical storage of hydrogen. NiMH cells also employ a positive electrode made from a nickel hydroxide active material. The negative and positive electrodes are disposed in an alkaline electrolyte and separated by a body of spacer material so as to form an electrochemical cell. Upon the application of an electrical potential across the NiMH cell, water is dissociated into one hydroxyl ion and one hydrogen ion at the surface of the negative electrode. The hydrogen ion combines with one electron and diffuses into the bulk of the hydrogen storage alloy. This reaction is reversible. Upon discharge the stored hydrogen is released to form a water molecule and release an electron.
The development of commercially viable NiMH batteries began in the 1980s with the improvement of the negative electrode materials which resulted from making them “disordered” as taught by Ovshinsky et al. in U.S. Pat. No. 4,623,597. Such negative electrode materials represented a total departure from other teachings of that period which advocated the formation of homogeneous and single phase negative electrodes. (For a more detailed discussion see U.S. Pat. Nos. 5,096,667; 5,104,617; 5,238,756; 5,277,999; 5,407,761; and 5,536,591 and the discussion contained therein. The disclosures of these patents are incorporated herein by reference.) Use of such disordered negative electrode metal hydride materials significantly increases the reversible hydrogen storage characteristics required for efficient and economical battery applications and results in the commercial production of batteries having high density energy storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
Further improvements in the performance of NiMH batteries resulted from improvements in the nickel hydroxide material incorporated into the positive electrodes of the batteries. In that regard, modifying and/or doping elements were added to the nickel hydroxide material so as to improve their structural and/or electronic properties. Some such compensating and/or doping materials include Co, Cd, Zn, Mg, and Ca among others. Such materials are disclosed in U.S. Pat. Nos. 6,228,535; Re. 34,752; 5,366,831; 5,451,475; 5,455,125; 5,466,543; 5,489,314; 5,506,070; and 5,571,636, the disclosures of which are incorporated herein by reference.
Charge capacity is a measure of how much electrical energy a battery is capable of storing and delivering. Consequently, charge capacity is a very important characteristic of any type of battery. As is evident from the prior art, as for example the prior art disclosed herein, significant strides have been made toward improving the charge capacity of rechargeable battery systems. However, it is also recognized in the art that performance characteristics of rechargeable batteries, including charge capacity, are adversely impacted when the battery systems are run under elevated temperature conditions. For example, in conventional NiMH batteries it has been found that operation under even modestly elevated temperatures such as 55° C. can reduce the run time of a battery by 35 to 55 percent compared to room temperature operation of the same battery. It is believed that this temperature-related loss of charge capacity is at least in part the result of undesirable electrode reactions, including the generation of oxygen at the surface of the cathode. Therefore, in addition to charge capacity, charge efficiency must also be taken into consideration in evaluating the performance of a battery system. In the context of this disclosure, “charge efficiency” is understood to refer to the amount of a battery's theoretical charge capacity which can actually be accessed under particular operating conditions. In this regard a battery having a high charge efficiency at elevated temperatures will be understood to manifest a charge capacity under such conditions, which is at most only moderately reduced from its charge capacity at lower temperatures. And conversely, a battery having a low charge efficiency at elevated temperatures will show a charge capacity which is much less than its charge capacity at lower temperatures.
Given the fact that rechargeable batteries often must operate under elevated temperature conditions, it will be understood that any improvements in their high temperature charge efficiency will be of great commercial significance. As will be explained hereinbelow, the present invention is based upon the finding that the high temperature performance of rechargeable alkaline batteries can be significantly improved if cobalt levels of materials comprising the positive electrode portion of the battery are selected so as to fall within particular ranges. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.